• journal_title：European Journal of Mineralogy
• Contributor：Gu Xiangping ; Xie Xiande ; Wu Xiangbin ; Zhu Guchang ; Lai Jianqing ; Hoshino Kenich ; Huang Jiwu
• Publisher：E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
• Date：2013-02-01
• Format：text/html
• Language：en
• Identifier：10.1127/0935-1221/2013/0025-2262
• journal_abbrev：Eur J Mineral
• issn：0935-1221
• volume：25
• issue：2
• firstpage：177
• section：Articles

Ferrisepiolite was discovered in the Saishitang copper skarn deposit in Xinghai County, Qinghai Province, China, as late-stage veinlets in copper-sulphide ores hosted in layered hedenbergite-andradite-actinolite skarn related to Indo-Sinian quartz diorite and Lower Permian metamorphosed clastic and carbonate rocks. Ferrisepiolite was formed in a highly oxidizing environment from low-temperature Fe-rich fluids and crystallized in cavities and fractures within the skarn-ore deposit. The mineral occurs in brown earthy and fibrous aggregates and shows brown to red-brown colour with strong pleochroism and 2nd order interference colours in a petrographic microscope. The measured refraction indices in white light for fibrous ferrisepiolite are: γ′ = 1.628(8), α′ = 1.592–1.620. The thermal analysis of ferrisepiolite reveals a lower dehydration temperature of structural hydroxyl than sepiolite and a small weight loss (0.1–0.9%) in the range 500–700 °C. The average chemical composition from wet chemistry, X-ray fluorescence spectrometry (XRF) and electron probe microanalysis (EPMA) is (Fe_1.84³⁺, Fe_0.51²⁺,Mg_1.56,Ca_0.05,Mn_0.02,Na_0.02)_∑=4(Si_5.79,Fe_0.21³⁺)_∑=6O₁₅.(O_1.60,OH_0.40)_∑=2·6H₂O for fibrous ferrisepiolite and (Fe_2.64³⁺,Fe_0.80²⁺,Mg_0.35,Ca_0.11,Mn_0.05,Na_0.05)_∑=4(Si_5.18,Fe_0.82³⁺)_∑=6O₁₅.(O_1.77,OH_0.23)_∑=2·6H₂O for earthy ferrisepiolite, which leads to the general formula: (Fe³⁺,Fe²⁺,Mg)₄(Si,Fe³⁺)₆O₁₅.(O,OH)₂·6H₂O. The powder X-ray diffraction (XRD) of ferrisepiolite is in agreement with that of sepiolite. The eight strongest lines of the pattern of fibrous ferrisepiolite are [d in Å (I)(hkl)]: 12.163(100)(110), 4.298(35)(131), 3.751(15)(260), 3.394(29)(400), 3.198(13)(331), 2.561(45)(191), 2.436(31)(212), 2.260(14)(391). Powder XRD and single-crystal electron-diffraction data show that ferrisepiolite is isostructural to sepiolite with orthorhombic unit-cell parameters (space group Pncn) a = 13.619 (8), b = 26.959 (26), c = 5.241 (7) Å, V = 1924.08 ų, Z = 4, Dcal = 2.51 g/cm³ for fibrous ferrisepiolite and a = 13.638 (9) Å, b = 27.011 (30) Å, c = 5.233(8) Å, V = 1927.58 ų, Z = 4, Dcal = 2.69 g/cm³ for earthy ferrisepiolite. Ferrisepiolite is the Fe(III)-dominant analogue of sepiolite with the substitution of Fe³⁺ and/or Fe²⁺ for Mg in the octahedral sites, compensated by substitution of Fe³⁺ for Si⁴⁺ in the tetrahedral sites and O²⁻ for OH⁻ in the sites of structural hydroxyl, accompanied by a contraction of the structure along the c-axis and an expansion along the a-axis.